BY AIR
The nearest airports are Krabi, Trang and Phuket. Krabi and Trang have direct road and boat connections to Ko Lanta, and are the best options if you're only visiting Ko Lanta. Flying into Phuket is a better option if you're intending to spend time in Phuket and nearby islands, or, since Phuket has many more international flights, if you don't want to have to organize a domestic flight to Krabi or Trang from Bangkok. As an alternative to Bangkok, there are daily flights to Krabi directly to and from Singapore..

BY CAR
Ko Lanta does not have roads connecting it directly to the mainland, but is served by car ferries. Take road 4206 off highway 4 about 20km south-east of Krabi airport. Drive to the end of the road, where there is a car ferry from Baan Hua Hin to Ko Lanta Noi. From the pier here, turn right and follow the "ferry" signs to the second car ferry to Ko Lanta Yai. Ferries operate 07:00-22:00
There are plenty of car taxi and minivan taxi services from Krabi and the nearby airport. A seat in a ten person minivan from the airport to Ko Lanta costs about 350 baht. These minivans usually don't have much room for luggage (although they can usually find room for one or two large suitcases together with backpacks for the rest of the passengers) so if you're not traveling light you might need to get a private taxi.
This is what the divemasters do. A private minivan taxi will usually be quoted as 2500 baht. It's possible to bargain a few hundred baht off this price but not much more or the tolls will consume too much of the price. Car taxis cost about the same.

BY BOAT
Ferry services run from Krabi, Ao Nang and from Phuket via Ko Phi Phi to Ko Lanta Yai's Saladan pier. Ferry services are subject to weather and to demand: many will not run at all during the monsoon season and at a reduced frequency during the low season. Many ferries have limited room for luggage. Often the ferries dock alongside each other, meaning that to get on and off passengers must clamber onto and across other boats with their luggage.
Services from Krabi are run by P.P. Family Co and depart Krabi pier at 10:30 and 13:30, taking 2 hours to reach Ko Lanta.